1. Field of the Invention
This invention relates to a safety device for a crane in which a boom is supported for swinging movement, and more particularly to a safety device for a crane of the type mentioned for setting a limit working region of the crane in accordance with a weight of a suspended cargo and performing a safety operation such as compulsory braking or stopping of the crane or alarming in response to such limit working region.
2. Description of the Related Art
Generally, a crane in which a boom is mounted for swinging movement includes such a safety device as will automatically stop the crane compulsorily when a working condition exceeds a safe region in order to prevent buckling, tipping and so forth of the crane.
Conventionally, in such device, allowable conditions are set equally over the entire region of 360 degrees irrespective of a swinging angle of the boom, but outrigger jacks provided on the crane may not always be projected completely, and depending upon a working location such as a narrow road, the amounts of projection of some of the outrigger jacks may be different from those of the remaining outrigger jacks. In such an instance, it is necessary to change the allowable conditions also in accordance with the swinging angle of the boom.
An arrangement is disclosed in Japanese Patent Laid-Open No. 57-27893 wherein a working condition of a crane is detected every moment and a rated load of the crane is determined from the thus detected value and preset values of the suspending capacity of the crane stored in a memory for various conditions of the crane, and a safety operation is performed based on comparison between the rated load and an actual load.
Since a calculated rated load and an actual load are compared with each other in this manner, the arrangement is convenient for the determination whether or not a current load can be suspended with a current length of the boom and a current working radius kept fixed. However, it is difficult to recognize such a limit working region to which position the boom can be swung with the load suspended thereon. Accordingly, it is substantially impossible to control a safety operation in which a limit working region may be exceeded by swinging movement of the boom.
Another arrangement is disclosed in Japanese Utility Model Laid-Open No. 62-89289 wherein an allowable swinging range of a boom is stored for each of the amounts of projection of individual outrigger jacks, and swinging movement and driving of the boom are stopped when an actual working condition exceeds the allowable swinging range.
In this arrangement, the boom is braked at an instant when the boom exceeds an allowable working region by its swinging movement. However, since inertial force by the swinging movement acts upon the boom, the swinging movement cannot be stopped quickly, and actually, even if the boom is braked, it will make further swinging movement over a certain angle. Consequently, the position at which the boom is stopped will actually come outside the limit working region. Besides, if urgent stopping is attempted with the intention of stopping the swinging movement in a period of time as short as possible, the suspended cargo will swing wildly due to the influence of inertial force, which will deteriorate the safety.